In recent years, with the advance of the communication technology, mobile phones have been widely used. In future, transmission of moving images and a larger volume of information are expected. On the other hand, through reduction in weight of personal computers, mobile personal computers have been produced. Information terminals called PDA originated in electronic notebooks have also been produced in large quantities and widely used. In addition, with the development of display devices, the majority of such portable information equipment are equipped with a flat panel display.
Recently among flat panel displays, manufactured for productization has been a display device having a thin film transistor (hereinafter, referred to as TFT) using a polycrystalline semiconductor crystallized at a temperature of 600° C. or less, which is rather low as compared to the conventional condition of 1000° C. or more. By the TFT using a polycrystalline semiconductor crystallized at a low temperature, not only a pixel but also a signal line driver circuit can be integrally formed around a pixel portion, which makes it possible to realize downsizing and high definition of a display device. Thus, such a display device is expected to be more widely used in future.
As for the display device having a TFT using a polycrystalline semiconductor crystallized at a low temperature, a display device in which a light-emitting element, particularly an organic EL element, is used as well as a liquid crystal display device has been developed. In addition, as for the display device in which an organic EL element is used, a passive matrix display device has been developed and produced for display devices of mobile phones, car stereos, and the like.
FIG. 2 schematically shows a conventional passive matrix display device. The display device shown in FIG. 2 has a pixel portion arranged in the center of a substrate 201 made of glass or the like. The pixel portion has light-emitting elements, column signal lines, and row signal lines arranged therein. A column signal line driver circuit 202 for controlling the column signal lines is disposed on the upper side of the substrate 201. On the left side of the substrate 201, a row signal line driver circuit 203 for controlling the row signal lines is disposed. Furthermore, a controller 240 is disposed for controlling the column signal line driver circuit 202 and the row signal line driver circuit 203. Note that the column signal line driver circuit 202 and the row signal line driver circuit 203 are each composed of LSI chips, and connected to the substrate 201 through a FPC (flexible Printed Circuit) (e.g., see Patent Document 1).
(Patent Document 1) Japanese Patent Application Laid-Open No. Hei 9-232074
Referring to FIG. 2, operation of a passive matrix display device is described. First, a row signal line 220 in the first row is selected. A state of being selected here means that a switch 212 is connected to GND. Next, switches 208 to 211 of the column driver are turned ON. One end of each of the switches 208 to 211 is connected to constant current sources 204 to 207 respectively, and the other end thereof is connected to column signal lines 216 to 219 respectively. When the switches 208 to 211 are turned ON, currents output from the constant current sources 204 to 207 flow into light-emitting elements 224 to 227 via the switches 208 to 211 and the column signal lines 216 to 219. Then, passing through the light-emitting elements 224 to 227, the currents further pass through the switch 212 via the row signal line 220, and flow into GND. In this way, the light-emitting elements 224 to 227 emit light in response to the flow of current therethrough. Periods in which the switches 208 to 211 are turned ON vary from one another. Gray scale display is thus performed based on the length of periods in which the switches are turned ON. After the switches 208 to 211 are all turned OFF, the switch 212 of the row signal line driver circuit is connected to VCC. Then, a switch 213 is connected to GND, and this operation will be repeated. In the case where a switch of the row signal line driver circuit is connected to VCC, a reverse bias is applied to light-emitting elements of the row, so that no current flows, and no light is emitted.
The luminance of light-emitting elements 224 to 239, that is, the amount of current flowing in the light-emitting elements 224 to 239 can be respectively controlled by the current value of the constant current sources 204 to 207 of the column signal line driver circuit and the length of period in which the switches 208 to 211 are turned ON. FIG. 3 shows an example of the column signal line driver circuit. A constant voltage is generated first with a built-in constant voltage source 301. As the constant voltage source, a known band gap regulator or the like is frequently used. In addition, a power source with a small temperature coefficient is used. The generated constant voltage is converted into a current by an operational amplifier 302, a transistor 303, and a resister 304. Thus, a constant current with a small temperature coefficient can be generated. The current is reversed and duplicated to obtain plural currents by a current mirror circuit composed of transistors 305 to 309 and resisters 314 to 318 before being supplied to the column signal lines via switches 310 to 313.
A method for gray scale display of a light-emitting element is described hereinafter. In the column signal line driver circuit shown in FIG. 2, when there is no variation in the length of ON periods among the switches 208 to 211, only two gray scales can be obtained in this display device. An expression method of the gray scale in this display device is described with reference to FIG. 4.
A timing chart of a time gray scale method is simply illustrated in FIG. 4. In this example, a frame frequency is set at 60 Hz, and 3-bit gray scale is obtained according to the time gray scale method. When the frame frequency is 60 Hz, one frame period is 16.6 ms. The value obtained by dividing this frame period by the number of pixels in the perpendicular direction approximately equals one horizontal line period 401. In the case where the number of pixels in the perpendicular direction is 220, for example, one horizontal line period is 75 μs. In the above-mentioned method, when 90% of this horizontal line period is an image period (a period in which an image signal exists), the image period is 68 μs. In the case of performing 3-bit gray scale display, that is, display in 8 gray scales in this image period, the length of ON period of the switch, namely a lighting period 402 may be set in proportion to gray scales, as illustrated in FIG. 4. In FIG. 4, a period denoted by reference numeral 403 is a non-lighting period and a period denoted by reference numeral 404 is a blanking period.
In the time gray scale method, the gray scale can be expressed in the above-described manner. It is of course possible to express the same kind of gray scale in a color display device.
FIG. 5 shows an active matrix display device. Pixels of the active matrix display device in FIG. 5 are configured by switching TFTs 508 to 511, EL driving TFTs 512 to 515, storage capacitors 516 to 519, and EL elements 520 to 523. Operation thereof is described below.
When a gate signal line 505 connected to a gate signal line driver circuit 502 becomes high, the switching TFTs 508 and 510 are turned ON and image signals supplied from source signal lines 503 and 504 connected to a source signal line driver circuit 501 are input to the storage capacitors 516 and 518 and the gates of the EL driving TFTs 512 and 514. Then, by the driving TFTs 512 and 514, the amount of current according to the voltage value flows from a power source line 507 into the EL elements 520 and 522. The EL driving TFTs 512 and 514 serve as voltage-to-current converting elements here. When the gate signal line 505 becomes low, the switching TFTs 508 and 510 are turned OFF. However, charge is held in the storage capacitors 516 and 518 so that the EL driving TFTs 512 and 514 maintain the same state to keep supplying current to the EL elements 520 and 522. As above, an active matrix display device comprises pixels having memory performance, therefore light emission at the same state can be continued until next writing starts.
When a gate signal line 506 becomes high, the switching TFTs 509 and 511 are turned ON and image signals of the source signal lines are input to the gates of the EL driving TFTs 513 and 515 and the storage capacitors 517 and 519. Current flows into the EL elements 521 and 523 by the EL driving TFTs 513 and 515, so that the EL elements 521 and 523 emit light. (The above description is, for example, disclosed in Patent Document 2.)
(Patent Document 2) Japanese Patent Application Laid-Open No. 2002-108285
Among active matrix display devices, a display device using a current mirror circuit as shown in FIG. 6 has been also developed. This display device comprises pixels provided with current mirror circuits which are configured by TFTs 609 and 610, TFTs 611 and 612, TFTs 613 and 614, and TFTs 615 and 616. A luminance signal is supplied not with voltage but with current from a source signal line driver circuit 601 to source signal lines 603 and 604, and gate signal lines 605 and 606 are controlled by a gate signal line driver circuit 602. When switches 621 to 628 are turned ON, the current mirror circuits operate so that the amount of current according to an output current of the source signal line driver circuit flows into EL elements 629 to 632. Even when the gate signal line driver circuit turns OFF the switches, the TFTs 610, 612, 614, and 616 operate to keep supplying current to the EL elements 629 to 632 in the case where charge is accumulated in capacitors 617 to 620 (e.g., see Patent Document 3).
(Patent Document 3) Japanese Patent Application Laid-Open No. 2001-147659
The aforementioned conventional organic EL display device has the following problems. First, as for a passive matrix organic EL display device, there is a problem in that the number of pixels can not be increased much. A passive matrix EL display device comprises pixels having no holding function, therefore light emission can be held only momentarily. The value obtained by dividing one frame period by the number of column lines equals a light emission period. The number of column lines is inevitably increased and the light emission period becomes shorter with the increase in the number of pixels. Generally, one frame period is approximately 16.6 ms in view of a flicker, and in the case where pixels are equal to 176×RGB×220, a lighting time of one line is 75 μs. When the light emission period is short while lighting luminance is high like the above, a large amount of current is required to be supplied to an organic EL element of a pixel, leading the short life of the organic EL element and the increase in power consumption due to the increase in forward voltage. Since a lighting period of a practical passive matrix display device is frequently set at 250 μs or more, it is difficult to increase the number of pixels in the passive matrix EL display device.
On the other hand, an active matrix organic EL display device as shown in FIG. 5 comprises pixels having memory function, therefore an organic EL element of a pixel can keep lighting during one frame period. The problem such as a passive matrix display device does not occur. However, in the aforementioned active matrix display device, voltage held in a capacitor is converted into current by a TFT in the pixel, so that variations in characteristics of TFTs affect the converted current. Low temperature polysilicon TFT is formed through crystallization using linear laser light, therefore due to the variations, characteristics of the TFTs vary in striped shape. Consequently, there is a problem in that variations in luminance arise in striped shape.
In the case of the display device using a current mirror circuit as shown in FIG. 6, the aforementioned variations in luminance can be prevented when there is no variations in characteristics of a pair of TFTs 609 and 610 which configures the current mirror. Uniform characteristics of the TFTs 609 and 610 can be realized by increasing each TFT size. However, TFTs occupies larger areas in a pixel and the opening ratio is decreased in such display devices, which leaves a problem in that such display device using a current mirror circuit is not applicable to a small pixel.